Optical fiber is used as a transmission medium, both to transmit light from one point to another, as well as to modify the characteristics of light, for example as a gain medium in a fiber laser or an optical amplifier, or as a dispersion compensating medium in a telecommunications network. When an optical fiber is used to modify the characteristics of light rather than for transmission, it is often preferred that the optical fiber be contained in a relatively compact volume which will be incorporated inside an enclosed unit, typically with linear dimensions of less than 0.5 meter.
Typical fiber-lasers or amplifier fibers are formed of lengths of optical fiber between 10 millimeters and 100 meters, most commonly in the 1-30 meter range, and generally the minimum permissible bend radius of the fibers is 10-20 millimeters. Other specialty optical fibers typically fall into the same range of parameters. Depending on the application, optical fibers may release enough heat, for example over one watt, to require efficient heat removal. Furthermore, the optical fibers used in fiber lasers should preferentially be protected from movement or accidental damage during handling. Due to the fragile nature of optical fiber, there is a need to protect the fiber from external sources of stress, such as bending, pressure and strain, which can damage the fiber and/or cause degradation of a signal being transmitted via the fiber. For example, a fiber should not be bent sharply anywhere along its path. In addition to the possibility of breakage or fracture, if a fiber is bent past a critical angle, portions of transmitted light pulses will leak out, rather than being reflected within the fiber core, thereby attenuating the transmitted light pulses and degrading signal quality. Accordingly, it is necessary that a fiber be routed so that bends in the fiber are of a sufficient radius to substantially avoid occurrence of such light leakage. U.S. Pat. No. 6,980,726 in the name of Daoud et al., discloses an optical fiber bend limiter that prevents contaminates from entering an optical fiber closure.
Compact lengths of optical fiber are traditionally held in spools similar to spools of string, that is, exteriorly-wound coils around a central spool or mandrel, with multiple coils, side-by-side, thus requiring a spool with significant thickness to accommodate the multiple coils; or lengths of optical fiber can be formed into loose coils bound to themselves with tape, plastic tie wraps, or similar devices. For example, U.S. Pat. No. 6,738,554 in the name of Daoud et al., entitled Double Helical-S Fiber Tray, discloses a tray in which optical fiber is wound in an “S” configuration. In this patent the optical fiber is shown to have lengths of optical fiber resting upon other lengths of optical fiber in layers building up within the enclosure. However, these known methods and devices for accommodating optical fiber do not necessarily ensure efficient heat removal or protection from movement or accidental damage, and they are often not as compact as desired.
There is therefore a need for a compact device for holding, protecting, and heat-sinking unwanted heat within a length of optical fiber.
Heat build up and heat transfer within optical fibers becomes a particular concern in high power operation, for example at 100 W or higher. In these instances lasing within the optical fiber at elevated temperatures inherently leads to losses in efficiency. This is a particular problem with Yb-doped fibers, because the energy levels are so close together in this dopant. At elevated temperatures, the thermal population of the lower laser level is increased and the population of the upper state is decreased, leading to decreased inversion and efficiency. Hence, it is important to maintain the fiber core as close to room temperature as possible, and certainly under 70 degrees C. Another drawback to operating a fiber laser at elevated temperatures is that over time, fiber temperatures over the 70 C range can also degrade the polymer coatings on the fiber, and possibly the fiber core materials as well. In a fiber laser having hundreds of watts of output power, the fiber may be dissipating multiple watts of power per meter across a total surface area of typically only a few square centimeters per meter, so having minimal thermal resistance to a heat sink is imperative in order to keep the core temperature near room temperature. The fiber's cladding layers and polymer coatings have significant thermal resistance and can lead to a temperature rise of several degrees or tens of degrees C.
Potting compound, and even “thermally conductive” potting compound, generally has much lower thermal conductivity than a metal; thus the use of any substantial thermal thickness of potting compound can significantly increase the thermal resistance and raise the core temperature. From the standpoint of contamination and optical damage, it is also important to minimize the amount of organic material in the vicinity of the fiber and particularly the fiber tips. The presence of the fiber polymer coating is is by design present and must be contended with, but potting material will typically be a much greater volume of organic material and much less cleanly applied. In the hot operating environment at full power, the use of potting compound raises a substantial risk of contamination of the fiber end faces with volatilized organic materials. Thus potting compound is not a desired solution to the problem of excess heat within the fiber.
It is therefore an object of the invention to holding the fiber in a way that has excellent heat transfer directly into high-conductivity metal, and no added risk of contamination by organic materials.
It is an object of this invention to provide a fiber holder that will hold a length of optical fiber in a compact manner while ensuring that the bend radius of the length of optical fiber is at least a predetermined radius so as to avoid the possibility of breakage, of excess light leakage, and that will facilitate the heat sinking of heat from within the optical fiber to the holder.